Quorum sensing is a process of bacterial cell-cell communication that involves production and detection of secreted signaling molecules called autoinducers (AI). Quorum sensing allows bacteria to collectively regulate gene expression and thereby function as multi-cellular organisms. For example, the bioluminescent Gram-negative quorum-sensing bacterium Vibrio harveyi integrates information from three different diffusible autoinducers that together enable intra- and inter-species communication. The three V. harveyi autoinducers are AI-1 (3-hydroxybutanoyl homoserine lactone), AI-2 ((2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran-borate), and CAI-1 ((S′)-3-hydroxytridecan-4-one). These signals are detected by the sensor-kinase proteins, LuxN, LuxQ, and CqsS, respectively (FIG. 1A) (Henke, J. M., and Bassler, B. L. (2004b). J Bacteriol 186, 6902-6914). At low cell density, (i.e., in the absence of autoinducers), these sensor kinases autophosphorylate and transfer phosphate to the shared phospho-transfer protein, LuxU. LuxU transfers the phosphoryl-group to the DNA-binding response regulator, LuxO, which activates transcription of genes encoding five redundant small regulatory RNAs called the quorum regulatory RNAs (Qrrs) (FIG. 1A). The Qrrs destabilize the mRNA transcript encoding the master quorum-sensing regulator, LuxR. Therefore, under low-cell-density conditions, the bacteria do not display quorum-sensing behaviors. In contrast, at high cell density the three autoinducers accumulate and bind to their cognate receptors. These binding events switch the receptors to phosphatases, resulting in dephosphorylation of LuxO and termination of Qrr production. The luxR transcript is stabilized, leading to LuxR protein production (FIG. 1A). LuxR controls the genes in quorum sensing, e.g., genes required for bioluminescence, siderophore production, type III secretion, and metalloprotease production (Fuqua, C., Winans, S. C., and Greenberg, E. P. (1996). Annu Rev Microbiol 50, 727-751; Hammer, B. K., and Bassler, B. L. (2003). Mol Microbiol 50, 101-104; Henke, J. M., and Bassler, B. L. (2004a). J Bacteriol 186, 3794-3805; McFall-Ngai, M. J., and Ruby, E. G. (2000). Curr Opin Microbiol 3, 603-607; Miller, M. B., and Bassler, B. L. (2001). Annu Rev Microbiol 55, 165-199; Waters, C. M., and Bassler, B. L. (2005). Annu Rev Cell Dev Biol 21, 319-346).
AI-1 is an acyl homoserine lactone (AHL) type autoinducer and it is the strongest of the three V. harveyi signals and, thus, the major input controlling quorum-sensing-regulated behaviors. Typically, AHL autoinducers are detected by cytoplasmic LuxR-type transcriptional activators (note: these LuxR-type proteins are unrelated to V. harveyi LuxR, FIG. 1A). V. harveyi is unusual because all three of its autoinducers, including AI-1, are detected by membrane-bound sensor-kinase proteins (in the case of AI-2, however, an additional periplasmic binding protein LuxP is required in conjunction with the membrane-bound two-component protein LuxQ). AI-1 is also the defining member of a growing family of recognized AHL type autoinducers that interact with membrane-bound sensor-kinases like LuxN, rather than with cytosolic LuxR-type proteins (Freeman, J. A., et al. (2000). Mol Microbiol 35, 139-149; Jung, K., et al. (2007). J Bacteriol 189, 2945-2948; Timmen, M., et al. (2006). J Biol Chem 281, 24398-24404). There are currently 11 LuxN homologs in the National Center for Biotechnology Information (NCBI) database, but nothing is known about how AHLs interact with this important class of receptors (FIG. 2).
Bacteria that use the AI-1 signaling factor associate with higher organisms, i.e., plants and animals, at some point during their life cycles. Some examples include Pseudomonas aeruginosa, Erwinia carotovora, Pseudomonas aureofaciens, Yersinia enterocolitica, V. harveyi, and agrobacterium tumefaciens. P. aeruginosa is an opportunistic pathogen in humans with cystic fibrosis. E. carotovora infects certain plants and results in soft rot disease. Y. enterocolitica causes gastrointestinal disease in humans and reportedly produces an autoinducer. P. aureofaciens synthesizes antibiotics under autoinducer control that block fungus growth in the roots.
Quorum sensing takes place not only among luminous marine bacteria like V. harveyi, but also among pathogenic bacteria where it regulates the production of virulence factors. Thus, it would be an advance to identify compounds useful for controlling pathogenic bacteria, and for augmenting traditional antibiotic treatments.